Intrinsically Linked

The camshaft drive mechanism

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In the early days of the automobile, different theories relating to almost every mechanical aspect of the engine were tested as inventors and manufacturers tried to work out the best, most reliable, most cost-effective and powerful technologies. Many ideas were rejected, but some became the norm throughout the industry. Though both two- and four-stroke engines were represented in the marketplace, the four-stroke Otto cycle design gained favor around the world. It became the standard-bearer for internal combustion engine theory.
Still, it was not the only engine available. Many within the engineering community refer to a two-stroke engine as a Clerk cycle engine. The first successful adaptation of this mechanical theory is attributed to a Scottish mechanical engineer, Dugland Clerk, who passed away in 1932.
One of the main differences between the two different engine cycles is the use of an eccentric camshaft to operate the valves in an Otto cycle design. In contrast, in its simplest form, a two-stroke Clerk cycle engine used reed-style valves that exposed ports in lieu of the mechanically operated poppet valve.
Thus, in a Clerk engine, the camshaft needs to be driven, and since it must be synchronized with the position of the piston, it is operated via the crankshaft.
There were and still are two-stroke engines that use poppet valves, which enjoy many of the features of a four-stroke design, but they are not found in automobiles today; they are the result of advanced development subsequent to Mr. Clerk's demise.
A drive is needed to make the linkage between camshaft and crankshaft work, because the camshaft (along with the distributor) makes one complete revolution for every two full turns of the crankshaft. Regardless of the method linking the two, the crankshaft will always make two revolutions for the camshaft's one.
There are three distinct methods for operating the camshaft from the motion of the crankshaft. The only difference in the approaches is in how the two are connected. The cam can be operated via a geartrain (gear-to-gear), rubber belt or a metal link chain. As with any mechanical apparatus, each application has both benefits and undesirable characteristics.
Geartrain
The camshaft is operated through direct gear-to-gear contact with the crankshaft through a set of intermediary gear(s) or via a shaft with helical gears on either end. This arrangement is still very common in large, heavy-duty diesel engines.
In most applications, the gears use helical teeth to ensure smooth and quiet operation. This quiet operation is the result of the progressive engagement characteristics of inclined teeth--one pair remaining in mesh until the following pair is partially engaged. To further reduce the noise from a geartrain drive, different materials for the gears themselves have been used over the years. Some applications used aluminum instead of steel, while composite gears were used as late as the mid-1980s on the General Motors 2.5-liter Tech-IV engine (a modern version of the Iron Duke from the 1962 Chevy Nova). That engine used a gear drive for the camshaft; it was rejected by the public for sounding too much like a diesel even though it was a spark-ignition powerplant.
Chain Drive
The chain-driven camshaft has been the most popular approach over the years. It employs at least two gears, one on the crankshaft snout and the other on the front of the camshaft. They are spaced some distance from one another and are connected with a metal link and roller-style chain that looks a lot like a bicycle chain.
In 1880, Hans Renold invented and patented a means to transmit power with a bush roller chain. It rigidly connected the side plates of the inner link pairs with bushings, which would oscillate in the bearing pins connecting the side plates of the outer link pairs, and which rotated the rollers that engaged the sprocket of the gear. A later, alternative design for cam-drive applications was the inverted-tooth chain; that was also invented by Renold, but in the 1920s.
Sprockets for both roller and inverted tooth timing chains are usually made from steel or cast iron. Many American engine manufacturers employed an inverted-tooth timing chain that was married to an aluminum alloy camshaft sprocket with nylon teeth. This was done for noise reduction.
Rubber Belt With Teeth
Similar in design and theory to the chain drive, the rubber belt system employs pulleys instead of two gears. The rubber belt rides in each pulley, one on the crankshaft and the other on the camshaft. In most applications, there is also either an idler pulley that is used to maintain belt tension or a separate tensioner along with an idler pulley.
The first production automotive use of a rubber tooth timing belt was in 1963 by the Hans Glas Company of Germany, but the belt design and manufacturing process was invented some 25 years earlier in America. In 1938, Richard Case developed this system for the synchronous needle and bobbin drive employed in sewing machines while employed the at U.S. Royal Rubber Company (which became Uniroyal).
To provide a positive timing drive--as opposed to a non-positive friction drive (like the traditional fan belt)--the teeth engage with axial grooves in the peripheries of the pulleys on the crankshaft and camshaft. Tooth profiles are based on either trapezoidal or semi-circular forms, which permit a smooth rolling motion upon engagement and disengagement of the belt with the pulley that is similar in action to the natural behavior of a gear.
Inherent Characteristics
When discussing which approach to use for a camshaft drive, we must pay attention to the crankshaft and the dynamics it is exposed to.
During the expansion of the flame during combustion, a number of things happen. Along with the reciprocating motion of the piston and connecting rod, the crankshaft is infused with a large amount of torsional vibrations and pulses. It's best to let the crankshaft, with its mass and vibration damper, absorb this harmonic, rather than transmitting it to the valvetrain. There will always be a small amount of harmonics transmitted to the camshaft, but the job of the engineer is to limit this effect.
Another concern is the phasing of the camshaft and, in turn, the valve events, to the proper position of the crankshaft. If the camshaft phase or rotational setting changes, then the dynamics of cylinder filling and emptying will be impacted due to the difference in the valve events.
The method used to drive the camshaft will have a direct effect on both of these concerns.
The geartrain system is the worst in terms of transmitting harmonics and vibration from the firing pulses to the camshaft and, thus, the valve lifter, pushrod and ultimately the poppet valve itself. The metal-to-metal or even nylon gear interaction is an excellent conduit for harmonics to travel with little or no opposition. For this reason, the geartrain design is best suited to a low-RPM engine with a very heavy crankshaft to minimize harmonics and the impact they will have on the valvetrain.
However, the geartrain system offers superior camshaft phase retention due to the lack of wear in the gears. This is especially important since any movement in the cam phase will impact the distributor timing with a gasoline engine and the injection pump timing on a diesel. As an aside, this was an issue with the Oldsmobile diesel engine, since the Roosa-Master injection pump was run via the camshaft gear. As the timing chain stretched, pump timing would be corrupted and the engine would run poorly.
Even on a low-RPM engine, gear noise is an issue with this cam drive. That is why it lends itself so well to use with a diesel or competition engine, since sound quality is usually not a concern or is masked by other aspects of the engine. Another benefit of the gear drive is the lack of movement or "walk" that the cam experiences while the engine is running.
The timing chain is an inexpensive means of providing reliable cam drive operation while limiting harmonics from the crankshaft. The chain itself has a tendency to absorb some vibration, but not all. A chain drive also eradicates a good deal of noise that would be created by a geartrain drive.
The main problem with a chain drive is the inherent wear that allows the chain to stretch and alter the cam phase and distributor timing. As the timing chain grows longer, the cam phase will retard with respect to the intake lobe centerline; consequently, engine performance, especially at low speeds, will be degraded. In order to cope with this, some designs employ a chain tensioner or double chain drive to keep the valvetrain load low and extend the useful service life of the chain before the camshaft phase becomes altered.
Another phenomenon that may occur from a fluctuation in engine speed is the excitation of the chain links, causing the chain to vibrate. To minimize this on some engines that are more prone to excitation due to the valvetrain load, length of the chain and the firing cycles of the crankshaft, a tensioner with rubber shoes is employed to quell the induced harmonic.
A rubber belt drive offers many advantages over the other designs. It has an inherent quietness and smooth operation, is lightweight and requires no lubricating oil like a geartrain or timing chain would. It also does a superior job of absorbing and minimizing the transference of any harmonics from the engine into the camshaft. On the other hand, it takes up more space because of its required width and the incorporation of either multiple idler pulleys along with a tensioner pulley. But its main pitfall is its limited service life. A rubber drive belt must be changed at a prescribed service interval, while a timing chain or especially geartrain drive will usually last the life of the vehicle.
With the advent of overhead cam engines in America, the rubber timing belt gained popularity, becoming the main cam drive design. Reliability became a concern, though, since many engines had no piston-to-valve clearance and if the belt broke, severe damage would occur. Such a design, where the belt could break and cause damage, was called an interference valvetrain; an engine that will not hurt itself if the belt fails at high speed is called a "freewheeling" engine. The interference valvetrain was very common on foreign-made engines. It was usually not found when Detroit used a rubber drive belt.
The problems of the interference valvetrain, and the high level of effort required to change a timing belt on a front-wheel-drive car, means that most manufacturers have reverted to using a timing chain on their OHC engines. Once again, the industry has come full circle.

This article originally appeared in the June, 2013 issue of Hemmings Classic Car.